English: Twin paradox employing a rocket following an acceleration profile in terms proper time τ: Phase 1: a=0.6, τ=2; Phase 2:a=0, τ=2; Phase 3-4:a=-0.6, 2τ=4; Phase 5:a=0, τ=2; Phase 6: a=0.6, τ=2. The blue numbers indicate the coordinate time "T" in the inertial frame of the stay-at-home-twin, the red numbers the proper time of the rocket-twin,and "a" is the proper acceleration. The thin red lines represent lines of simultaneity in terms of the different momentary inertial frames of the rocket-twin. For a similar diagram in terms of coordinate time, see TwinParadoxProperAcceleration.svg